Issues with Transitions - The Chevron transition doesn't work

I'm using FCP X version 10.4.8 and having an issue with the Chevron transition. I added this transition to my project at an early stage and it plays fine. For some reason I am now unable to add an additional Chevron transition anywhere else in the project. I can see the transition occur if I slowly scrub through that point on the timeline, however, it doesn't playback in real time. The timeline video plays a if there is a simple edit between two clips. All of my other transitions play fine and new ones can be added to the timeline.

I deleted my preferences and relaunched FCP X. The problem persists.

I started a new library, created a short timeline and added three clips. I dropped the Chevron in between the clips and the problem is still there.

I'm editing off a 3 TB external media drive and still have 663 GB free.

Are there technical issues with this version of FCP X or a problem with the Mac computer I'm using?

What could be causing this problem with Chevron and how can I fix it?